electron-capture delayed fission (ecdf) in the pb region
DESCRIPTION
178. Tl. 150 ms. a. Electron-Capture Delayed Fission (ECDF) in the Pb region. 200,202 Fr. 192,194, 196 At. Andrei Andreyev. 186,188 Bi. 178, 180, 182 Tl. Z=82. ISOLDE workshop 18th November 2008. Collaboration. Jarno Van De Walle RILIS & ISOLDE. Andrei Andreyev Nick Bree - PowerPoint PPT PresentationTRANSCRIPT
1
Electron-Capture Delayed Fission (ECDF) in the Pb region
Andrei Andreyev
ISOLDE workshop 18th November 2008
178Tl150 ms
178,180,182Tl
Z=82
200,202Fr
192,194,196At
186,188Bi
2
Collaboration
Stanislav AntalicAndrei Andreyev
Nick BreeThomas Cocolios
Jan DirikenJytte Elseviers
Mark Huyse Paul Van den Bergh
Piet Van DuppenMartin Venhart
Robert PageKatsuhisa Nishio
U. Koster (ILL, Grenoble, France)S. Franchoo (IPN, Orsay, France)S. Vermote, C. Wagemans (University of Gent, Belgium)M. Veselský (Slovak Academy of Sciences, Bratislava, Slovakia)I. Tsekhanovich (Manchester University, UK)
Jarno Van De WalleRILIS & ISOLDE
3
• Electron-Capture Delayed Fission– what it is and why?
• Earlier ECDF studies in the U and Pb regions
• ECDF of 192,194At at SHIP: cold fission of 192,194Po
• ECDF of 180Tl at ISOLDE: first ever fission study at ISOLDE?
• Plans
Outlook
4
Electron-Capture Delayed Fission (ECDF, T1/2(ff)=T1/2(EC))
Discovery: parent isotopes 232,234Am(1966, Dubna)
PECDF depends strongly on:
• QEC of the parent: the higher QEC, the larger the PECDF
• Bfis of the daughter: the lower Bfis, the larger the PECDF
• Actually, QEC-Bfis is important
deformation
A,Z
QEC
Bf
A,Z-1
•2 step process: EC decay of a parent (A,Z) nucleus populates an excited state in the (A,Z-1) daughter, which then might fission (in competition with the decay to the g.s.)•Low-energy fission! (E*~5-10 MeV)•12 cases know so far (neutron-def. Uranium region)
EC
ECDF
NEC
NECDF
EC-delayed branch
PECDF= NECDF
NEC
5
ECDF Probability: Feeding Part & Decay Part
NECDF
NEC
PECDF= =
(QEC-E)2 – Phase factor for EC decay
S(E) – -strength function (nuclear matrix element)
QEC
(QEC-E)2S(E) dEf(E)tot(E)
(QEC-E)2S(E) dE
0
0
QEC
-ratio of the fission and total widths of excited levels in daughtern is not important for neutron-deficient nuclei)
f
f+
f
tot
=
f= {1+exp[ ]}-12(Bf-E) hf
12
-inverted parabola approximation D.L. Hill and J.A.Wheeler
=
9.710-7 T4 exp(E/) 2
, – level density, T - temperature
Measurement of PEDCF allows to deduce Fission Barrier Bf
6
-delayed Fission and r-process Nucleosynthesis
Trans-Uranium elements?Cowan et al, Phys. Rep. 208 (1991) 267
End point of r-process?I.Panov et al, NPA747 (2005) 633
Where the fission process will stop the r-process?
r-process
r-process to the region with A>250
Fission cycling: Fission products can serve as seed for the r-process! I. Panov et al, NPAA747 (2005) 633
Fission AA/2 ~125
Fission: dfn-inducedsf
r-process Fission: df n-induced, sf
Need fission data for very heavy exotic nuclei!
7
Why ECDF studies?
•Theoretical models for Bf have been ‘tuned’ by using these data• Large discrepancies between different models for n-def. and n-rich nuclei
Available data on fission barriers, Z ≥ 80 (RIPL-2 library http://www-nds.iaea.org/ripl-2/ )
N/Z~1.55-1.59
•ECDF gives access to nuclei with very exotic N/Z ratios: e.g. N/Z(178Hg)=1.225•Unexpected properties, e.g. mass-distributions, cold fission…?
Region of our interest: A~180-200 N/Z~1.22-1.3
126
82
• Experimentally fission barrier are known only in the vicinity of the -stability line (e.g. N/Z(238U)=1.59)
• Must study isospin dependence of Bf values
Neutron Number
Bf [
Me
V]
Po
A. Mamdouh et al. NPA679 (2001), 337
LDM
TF
ETFSI
8
90 100 110 120 130 140 150Neutron Number N
Pro
ton
Num
ber
Z
208Pb80
90
100
Courtesy P. Möller
QEC values: P. Möller et al., ADNDT masses (1995), based on FRDM (1992)
Bf values: P. Möller et al., submitted to PRC (2008)
238U
Calculated Energy Window for EC-delayed Fission
9
ECDF in trans-U region
PD
F
• 12 known ECDF cases in trans-Uranium region (all odd-odd!)
• Relatively low QEC and Bf values (3-5 MeV)
PD
F
PD
F
180Tl ?180Tl ?
242Es (48)
248Es (4)
246Es (1)
QEC [MeV] QEC-Bf,shell [MeV]
PD
FWhy odd-odd EC-decaying parents?• Larger QEC in comparison with e-e and o-e neighbors (odd-even effect) • Even-even daughters are more fissile (specialization energy)
176 178 180 182 184 186 188 190 192456789
1011121314151617
Tl-Hg
Bf(Hg)
QEC(Tl)
10
ECDF possibleQEC > Bf
bEC>1%
QEC vs Bf values in Pb region (look for cases QEC > Bf)
Red lines: Thomas-Fermi Fission Barriers, W.D. Myers, W. Swiatecki Phys. Rev. C60 (1999) 014606
Black lines: QEC-values, P. Moller et al. At. Data and Nucl. Data table 59 (1995) 185
176 178 180 182 184 186 188 190 192456789
1011121314151617
184 186 188 190 192 194 196 198
4
6
8
10
12
14
16
18
20
186 188 190 192 194 196 198 200
3456789
101112131415161718
190 192 194 196 198 200 202 204 206
456789
10111213141516171819
192 194 196 198 200 202 204 206 20823456789
101112131415161718
200 202 204 206 208 210 212 214 216
0123456789
101112131415
206 208 210 212 214 216 218 220
2
4
6
8
10
12
14
198 200 202 204 206 208 210 212 214
2
4
6
8
10
12
14
16
18
180 182 184 186 188 190 192 1940
2
4
6
8
10
12
14
16
18
Tl-Hg
Po-Bi
E [
MeV
]
E [
MeV
]
E [
MeV
]
At-Po
Rn-At
Ra-Fr
Ac-Ra
Fr-Rn
Bf(Hg)
QEC(Tl)
Pb-Tl Bi-Pb
Mass Number
QEC(At)
Bf(Po)
QEC(Fr)
Bf(Rn)
Bf(Pb)
QEC(Bi)
11
Velocity Filter SHIP (GSI, Darmstadt)
SHIP: Separation time: 1 – 2 μs
Transmission: 20 – 50 %
Background: 10 – 50 Hz
Det. E. resolution: 18 – 25 keV
Det. Pos. resolution: 150 μm
Dead time: 3 – 25 μs
31 cm
Rotating targets ~400-600 g/cm2
1 pA of 52Cr,58Ni~61012 pps
12
SHIP Detection System
A.N. Andreyev et al. ‘Conversion electron and decay spectroscopy at SHIP’, NIM A533, 416 (2004)
BOX of 6 Si detectors PSSD Ge Clover
, X-raysEsc. ff
Recoils
e
ff
Measure efficiently all possible decays:
• particle decay (, , protons, fission) E=0.1-250 MeV
• gamma decay E=10-4000 keV
• internal conversion electrons E=50-500 keV
•3 Time-Of-Flight detectors
•STOP detector – 16 position sensitive Si strips (35×80mm), pos. resolution FWHM=150 μm, energy resolution 14 keV
• 6 BOX Si detectors – for and escaping particles with a solid angle 80% of 2π
• GAMMA detectors – large-volume Clover detector for x rays or rays in coincidence with ’s
• VETO detector – reduces background
13
1
10
102
103
104
105
1
10
102
103
104
105
PSSD total
PSSD pause
EPSSD [MeV]0 45 90 135 180 225 270
56Fefull energy
Recoils
Fissions (66 events)
56Fe+141Pr194At+3nI(56Fe)~600 pnA
86103 nuclei66 fission events (pause)
Co
unt
s
First observed in the 52Cr+144Sm194At+pn reaction (SHIP,2006)16 fissions in pause
Identification based on:• Half-life T1/2,(194At)=280(20) ms T1/2,fiss(194At)=300(60) ms• Excitation functions• Cross-irradiation (56Fe,52Cr)• Double fold ff events (BOX)• Coincidences with gammas
Unambiguous identification of ECDF in 194At, SHIP(GSI)
A.Andreyev et al, paper in preparation
Scattered projectiles
On OFF5ms 15 ms
4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 5.0 5.10.01
0.1
1
10
100
1000
Cro
ss-s
ec
tio
n [
nb
]
Beam energy [MeV/u]
195At()
194At ()
194At (ff)
193At ()
56Fe+141Pr197At*
14
PECDF for 192,194At
192At
194At
-2 -1 0 1 2 3 4
1
10-1
10-2
10-3
10-6
10-4
10-5
LogPECDF=0.81475(QEC
-Bf)-4.2196
PE
CD
F
QEC
-Bf,Shells [MeV]
242Es
244Es246Es
248Es
250Md
232Am
234Am240Bk
238Bk
228Np
QEC and Bf values are from the TF model
Largest PECDF values ever obtained!
15
TKE(194At) ≈ 159 (7) MeV
(corrected for PHD)
Total Kinetic Energy in the EC-delayed fission of 194At
TKE [MeV]
TKE: Add up the energies of 2ff from the PSSD and BOX detectors
Co
unt
s
75 125 175 225 0
BOX of 6 Si detectors PSSD
ff1ff2
1200 1300 1400 1500 1600 1700
160
180
200
220
240
260Q-value (most probable mass split) TKE, Viola fit
194Po
RfNo
Md
CfFm
CmPuU
192Po
aver
age
or
mos
t p
roba
ble
TK
E [
MeV
]
Fissility Z2/A1/3
246Fm
252 No
Cm
252,256Cf
238U
246,250
236,240,244Pu
------ Q-value (most probable mass split)
Cold fission of 192,194Po!?
16
FIRST FISSION STUDY AT ISOLDE!?FIRST FISSION STUDY AT ISOLDE!?
December 1938: O. Hahn and F. Strassmann : discovery of nuclear fission
From 16th October 1967 on: ISOLDE operation
40 years later….June 2008: first ever fission study at ISOLDE!? : IS466 experiment
17
IS466: ECDF ofIS466: ECDF of 180 180Tl isotope at ISOLDE (31 may-6 June Tl isotope at ISOLDE (31 may-6 June 2008)2008)
Annular Si Si
pure 30 keV Tl beam from RILIS+ISOLDE
Setup: Si detectors from both sides of the C-foil
• Simple setup & DAQ: 4 PIPS (1 of them – annular)• Large geometrical efficiency (up to 80%)• 2 fold fission fragment coincidences• ff-gamma coincidences• Digital electronics (5 DGF modules)
C-foils20 g/cm2 Si detectors
176 178 180 182 184 186 188 190 1924567
89
101112
1314
151617
Tl-Hg
Bf(Hg)
QEC(Tl)
Tl/Hg mass
E, MeV
30 keV beam from ISOLDE
SiAnnular Si
ff
ff
C-foil
MINIBALL Ge cluster
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IS466: EC and decay of 180Tl
5600 5800 6000 6200 6400 66000
10000
20000
30000
40000
50000
60000
180Tl
180Hg
176Pt
Alpha energy [keV]
Si detector
Co
unt
s
• Very clean spectra: only 180Tl and its decay products! (first on-line use of SSL)
180Hg
EC
180Tl
176Pt
176Au
176Au
• Negligible direct 180Hg production (bEC(180Tl))
100 times statistics than in previous works
• New detailed decay scheme of 180Tl
• Detailed data (~10 new lines in 176Au)
• Correct low-energy level scheme of 176Au
Due to negligible direct production of 180Hg
PECDF(180Tl)=
PECDF(180Tl)=5(1)10-5 (PECDF=310-(7±1) )
Nfission(180Hg) N(180Hg)bEC(180Hg)
ECDF
19
IS466: ECDF of 180TlBefore the IS466 experiment: How 180Hg (Z=80, N=100, N/Z=1.25) fissions?
SYMMETRICAL mass split in two semi-magic 90Zr(Z=40,N=50, N/Z=1.25)?
IS466: NO! ASYMMETRICAL energy (thus, mass) split!
Eff1-Eff2 coincidences ~330 events
30 40 50 60 70 80 90 100
3040
5060
7080
90100
0
5
10
15
20
25
Eff1 [MeV]
Eff2 [MeV]
Singles 1300 ff
From comparison of TKE and Q-values Cold fission of 180Hg!
20
Future ECDF studies in the Pb regionFuture ECDF studies in the Pb region
178Tl150 ms
178,180,182Tl
Z=82
200,202Fr
192,194,196At
186,188Bi
Identification of new ECDF nuclei and detailed studies (e.g. Bf, TKE…)
ISOLDE:•180Tl – Beta strength function measurements with TAS•180Tl – HFS scan with RILIS : search for 2 isomeric states • 180Tl – mass measurement at ISOLTRAP•178,182Tl – ECDF experiments at ISOLDESHIP:•186,188Bi •200,202Fr, 192At
21
Thank you!Thank you!
22
TKE(194At) ≈ 159 (7) MeV
(corrected for PHD)
Total Kinetic Energy in the EC-delayed fission of 194At
TKE [MeV]
TKE: Add up the energies of 2ff from the PSSD and BOX detectors
Co
unt
s
75 125 175 225 0
BOX of 6 Si detectors PSSD
ff1ff2
1200 1300 1400 1500 1600 1700
160
180
200
220
240
260Q-value (most probable mass split) TKE, Viola fit
194Po
RfNo
Md
CfFm
CmPuU
192Po
aver
age
or
mos
t p
roba
ble
TK
E [
MeV
]
Fissility Z2/A1/3
246Fm
252 No
Cm
252,256Cf
238U
246,250
236,240,244Pu
------ Q-value (most probable mass split)
252Cf(sf): Q[252Cf-(142Ba+110Mo)]=219 MeV <TKE>=185 MeV(Q-TKE)=34 MeV
194Po(ECDF): Q[194Po – (98Mo+96Mo)]=166 MeV <TKE>=157(7) MeV(Q-TKE)=9(7) MeV (+E* after EC)
192Po(ECDF): Q[192Po – (96Mo+96Mo)]=165 MeV <TKE>=170(10) MeV(Q-TKE)= - 5(10) MeV (+E* after EC)
Cold fission of 192,194Po? (no neutron emission during fission!)
23
-3 -2 -1 0 1 2 3 41E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0.01
0.1
1
Tl ???
250Md
228Np
232Am
234Am
238Bk
240Bk
246Es
242Es
244Es
PD
F
QEC-Bf,shell [MeV]
248Es
ECDF probability in the Pb region
190 192 194 196 198 200 202 204 206
4
6
8
10
12
14
16
18 At-Po
198 200 202 204 206 208 210 212 214
2
4
6
8
10
12
14
16
18
Fr-Rn
Mass Number
E [
MeV
]
E
[M
eV]
Large QEC and QEC-Bf=3-4 MeV Possibility to reach nuclei with 10-100% ECDF probability!
180
24
Low-Energy ECDF data (232,234Am as examples)
H. L. Hall et al., PRC42 (1990) 1480• Fission barriers and their isospin dependence• Energy distributions of fission fragments, thus TKE• Mass distributions of fission fragments• Gamma multiplicities for fission fragments• Neutron multiplicities for fission fragments
K X rays in coincidence with ff
ALL THIS FOR VERY EXOTIC NUCLEI WHICH DO NOT FISSION SPONTANEOSLY!
25
QEC(At) and Bf(Po) in different models
190 192 194 196 198 2005
6
7
8
9
10
11
12
13
Q
EC o
r B
f [M
eV]
At/Po Mass Number
Qec_At_TF Qec_At_MN_FRDM Qec_At_MN_FRLDM Bf_Po_TF_FLDM Bf_Sierk_shells_FLDM Qec_DF Audi2003 Qec_AT_EFTSI
Bf
QEC
* exp QEC
26
-Delayed Fission (DF, T1/2(ff)=T1/2())
deformation
A,Z
Q
Bf
A,Z+1
•2 step process: decay of a parent (A,Z) nucleus populates an excited state in the (A,Z+1) daughter , which then might fission (in competition with the decay to the g.s. and/or neutron emission)
•Low-energy fission! (E*~5-10 MeV)
•Neutron-rich nuclei (only 5 candidates known so far)
ff DF
27
Example: Fission barriers – what do we know about them?
• Experimentally fission barriers Bf are known only in the vicinity of the beta stability line (e.g. N/Z(238U)=1.59)• Theoretical models for Bf have been ‘tuned’ by using these data Available data on fission barriers, Z ≥ 80 (RIPL-2 library http://www-nds.iaea.org/ripl-2/ )
N/Z~1.55-1.59
126
82
• For the r-process calculations we need fission data far away from stability: e.g. 260Po or 270U (N/Z>2!) – they might not be accessible in the Lab at all! – Use calculations?
N/Z>2
-2.2
28
Fission Barrier Calculations for the r-process nuclei
Neutron Number Neutron Number
Bf [
Me
V]
Full symbols – experimental data Lines – calculations (LDM,TF, ETFSI)
Po U
A. Mamdouh et al. NPA679 (2001), 337
LDM
TF
ETFSI
LDM
TF
ETFSI
• Good agreement between Bf,cal and Bf,exp for nuclei close to stability• Large disagreement far of stability (both on n-def. and n-rich sides)• Need measured fission data far of stability to ‘tune’ fission models
•Unfortunately, so exotic nuclei are not presently accessible by available techniques!
•That is why the underlying mechanisms and properties of beta-delayed fission (and of low-energy fission in general) have to be investigated by using alternative approaches and in other regions of the Nuclear Chart.
•According to semi-empirical estimates, the neutron-deficient nuclei in the U and Pb regions provide such a possibility via the ECDF decay
29
Fission Barrier Calculations for the r-process nuclei
Neutron Number
Full symbols – experimental data Lines – calculations (LDM,TF, ETFSI)
U
LDM
TF
ETFSI
• Good agreement between Bf,cal and Bf,exp for nuclei close to stability• Large disagreement far of stability (both on n-def. and n-rich sides)• Need measured fission data far of stability to ‘tune’ fission models
Bf [
Me
V]
Po
A. Mamdouh et al. NPA679 (2001), 337
LDM
TF
ETFSI
30
(Speculations?) Double-Magic Fission?
P. Moller (private communication): Most probably – NOT as a main channel• Most probable mass splits ~90/~90 (90Zr/90Zr N/Z=1.25)
• but what about very asymmetrical split at the wings? :
Schmidt et al., NPA 665 (2000) 221
How its daughter 178Hg(N/Z=1.225) would fission? Would it give ‘double-magic’ fission – two double-magic ff’s? very neutron-rich 78Ni (Z=28,N=50 N/Z=1.79) very neutron-deficient 100Sn (Z=50,N=50 N/Z=1)
One of the goals of ISOLDE proposal is ECDF of 178Tl
31
ECDF Probability: Fission/gamma competition
Measurement of PEDCF allows to deduce Fission Barrier Bf
e.g . H.V. Klapdor et al., Z.Phys.A292, 1979,249; D. Habs et. al. Z.Phys. A285 (1978), 53
-ratio of the fission and total widths of excited levels in daughtern is not important for neutron-deficient nuclei)
f
f+
f
tot
=
f= {1+exp[ ]}-12(Bf-E) hf
12
-inverted parabola approximation D.L. Hill and J.A.Wheeler
= 9.710-7 T4 exp(E/) 2
, – level density, T - temperature
QEC
PECDF=
(QEC-E)2S(E) dEf(E,Bf)tot(E)
(QEC-E)2S(E) dE
0
0
QEC
NECDF
NEC
~
32
Previous Studies in the Pb region (Dubna)Yu. A. Lazarev et al. Europhys. Lett. 4 (1987) 893; and Inst. Phys. Conf. Ser. No132 (1992) 739
“Most probable” candidates: 180Tl (PECDF=310-(7±1)), 188Bi,196At (no PECDF data) • Irradiations inside the cyclotron (no A,Z selection for products) • Rotating wheel system, thick effective targets (2 mg/cm2)• Cross-irradiations, apparent fis~15-50 pb• Mica detectors (fission tracks only)• Never confirmed (not in the Tables) • No continuation with these studies so far
PD
F
180Tl ?180Tl ?
QEC [MeV] QEC-Bf,shell [MeV]P
DF
176 178 180 182 184 186 188 190 192
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Tl-Hg
Bf(Hg)
QEC(Tl)
33
An example: Fission Barrier of 232Pu D. Habs et. al. Z.Phys. A285 (1978), 53
QEC
PECDF~(QEC-E)2S(E) dEf(E)
tot(E)
(QEC-E)2S(E) dE
0
0
QEC
f= {1+exp[ ]}-12(Bf-E) hf
12
Measured PECDf (232Am) =(1.3 )10-2 Bfis(232Pu)=5.3(4) MeV Assuming uncertainty of: QEC=±200 keV hf =±100 keV a factor of 3 in PECDF
+4
-0.8
Bf precision of ~7.5% - well comparable to direct methods!
34
Experimental information on fission - Low Experimental information on fission - Low energyenergy
- particle induced x - e.m. –induced E*~11 MeV
Region of ourinterest: A~186-200 N/Z~1.26-1.3
N/Z~1.55-1.59
35
Electromagnetically-Induced Fission In-flight (FRS, GSI)
A. Grewe et al. NPA614 (1997), 400
Sierk (FRLDM)
-EM fission
• Primary beam 238U at 1 AGeV• 1 g/cm2 Cu primary target • Separated RIBs from FRS• Pb secondary target• (El.fission)~2.1 b (234U)
Pashkevich
ActiveTarget
Fission Fragments
Identified SecondaryBeam E~620 AMeV
- particle induced+SF x -e.m. -induced
Fission from E*~12 MeV (E1 GDR)
For comparison: Bf(234U) ~ 6 MeV Bf(220Th) ~ 7.5 MeV Bf(218Ac) ~ 7.5 MeV
Thus, still fission from quite above the barrier!
In contrast, df is near (or sub)-barrier effect!
36
246Fm
252 No
Cm
252,256Cf
238U
246,250
236,240,244Pu
1200 1300 1400 1500 1600 1700
160
180
200
220
240
260Q-value (most probable mass split) TKE, Viola fit
194Po
RfNo
Md
CfFm
CmPuU
192Po
aver
age
or
mos
t p
roba
ble
TK
E [
MeV
]
Fissility Z2/A1/3